In radio communications systems, for example the second generation European Mobile Radio System GSM (Global System for Mobile Communication), information (speech, images or other data) is transmitted via a radio interface with the aid of electromagnetic waves. The radio interface relates to a connection between a base station and one or more subscriber stations, in which case the subscriber stations may be mobile stations or fixed-position radio stations. The electromagnetic waves are in this case emitted at carrier frequencies which are in a frequency band specified for the respective system. Frequencies in the frequency band around 2000 MHz have been provided for future radio communications systems, for example the UMTS (Universal Mobile Telecommunications System) or other third generation systems. Two modes are envisaged for this third mobile radio generation, with one mode being referred to as FDD (Frequency Division Duplex) operation, and the other mode being referred to as TDD (Time Division Duplex) operation. Both modes support a so-called CDMA subscriber separation method (Code Division Multiple Access).
In the case of radio stations, particularly in the case of mobile stations for digital mobile radio, which, for example, support the GSM or UMTS standard, one major precondition is the capability to set the transmission time of the transmission signal. This is necessary in order to comply with the time references specified in the respective Standard.
For example, the transmission time of a mobile station is determined by the base station to which the mobile station has been synchronized. The transmission time of the mobile station is in this case corrected with respect to the data received from the base station. No information may be added to or removed from the transmission signal during the correction of this transmission time. This is associated with the requirement that only relatively minor corrections may be carried out to a symbol to be transmitted, and that large sudden time changes during the correction of the transmission time of the transmission signal must be avoided.
Furthermore, the rate of change with which a correction must be carried out is specified in the respective Standard, and must be complied with during the correction. In order to match the transmission time of the transmission signal to the nominal transmission time as well as possible, it is necessary to minimize the phase fluctuations in the signals, for example the so-called jitter, which refers in general to phase fluctuations.
German Laid-Open Specification DE 198 58 358 A1 discloses a method for time synchronization of radio stations in a radio communications system, in which case the radio stations may be base stations or moving mobile stations. The matching between the radio stations is carried out alternately. A second radio station receives transmissions from at least one first adjacent radio station via a radio interface. This second radio station uses the received transmissions to determine a reception time, and compares this reception time with the transmission time of its own transmissions. The result of the comparison is transmitted as synchronization information to the first radio station, which once again matches its own transmission time to the transmission time of the second radio station, in the sense of the received synchronization information.
This method is complex, and both the transmission time and reception time must be changed and taken into account in each of the radio stations.
A further method for synchronization of the timebase of a receiving unit to the timebase of a transmitting unit in a telecommunications device is known from German Laid-Open Specification DE 195 25 426 C1. The receiving unit and the transmitting unit may each be a base station or a mobile station, and may be used for signal transmission. The base station and mobile station timebases, which are generally not the same as one another, are continuously synchronized while a connection is in existence, in order to neutralize this discrepancy. The transmitting unit transmits synchronization data, for example a synchronization word, for clock recovery, and data for synchronization of the receiving unit, in each channel time slot of a transmission frame in a data burst to the receiving unit. The data received in the receiving unit is compared until the correct synchronization data is identified. The actual synchronization bit contained in the received synchronization word is determined, and its position is compared by means of a comparator with the nominal synchronization bit, which is used as a reference bit in the receiving unit. If the two bit positions differ, the comparator produces an error signal. An error counting unit, which is connected downstream from the comparator, counts the error signals in a predetermined time period, which is governed by the clock frequency for data burst transmission, and by the number of counting steps of a frame counting device. The time sequence of the error signals that are determined is a measure of the difference between the timebases of the receiving unit and of the transmitting unit. The receiving unit uses the number of stored error signals to produce a correction signal, which is used for synchronization during reception during a connection, or during a connection interruption in the event of reception failure.
The synchronization of the timebases of the receiving unit and of the transmitting unit in the known method is relatively complex, and relatively inaccurate.